Look at these gauges. See the bubbles
in the glass? That's glycerin. What's it doing in there?

These type of gauges
are designed for use where shock or pulsation cause their premature
failure. Filling them with glycerin dampens the tube and
pointer movement. The fill also lubricates and protects the movement
and prolongs the life of the gauge.

So, if one of your gauges loses its
glycerin fill, don't worry so much. It will still read
pressure but the needle will be flickering and eventually? It
will fall right off. So have a replacement on hand if you find
a dry gauge.